1. Field of the Invention
My invention relates generally to means for acquiring and maintaining an optical communication link between two spatially isolated transceivers and, more specifically, to such means as used in satellite communication applications.
2. Description of the Related Art
The advantages of using optical means for forming a communication link between satellites include enhanced jamming resistance, transmitted data security, and low power consumption. To establish an optical communication link between two transceivers, where each transceiver is mounted on a satellite platform, at least one of the transceivers must actively scan some predefined solid angle, preferably a hemisphere of 2.pi. steradians. The transceiver initiating the communication link may be called the "searching transceiver" and the other transceiver may be called the "target transceiver," although in some systems each may be actively searching for the other. The searching transceiver may lock onto or "acquire" the target transceiver in response to a beam transmitted by the target transceiver or a reflection of its own transmitted beam returning from a "retro-reflector" on the target transceiver.
Acquisition can be accomplished by means of an active feedback controller in the target transceiver that adjusts the target optics to maintain the incoming beam incident upon the center of a sensor array. A quadrant sensor comprising four photodetectors disposed in a square pattern can be used. The relative intensities of light impinging upon each sensor are used to produce error signals in the horizontal (X) and vertical (Y) directions, relative to an origin lying in the center of the square sensor pattern. The error signals then are used to control servo-motors for adjusting the optics.
It is conceivable that the searching transceiver may never acquire the target transceiver without first obtaining precise boresight alignment between a narrow transmitted laser beam and a sensor. Once such precise boresight alignment is achieved, additional problems arise in maintaining the communication link alignment between the two moving satellites.
Some of these problems known in the art for acquiring and maintaining an optical communication link have been addressed in U.S. Pat. Nos. 3,566,126 issued to Lang et al. and 3,511,998 issued to Smokler. Lang uses a corner reflector as a type of retro-reflector mounted on a second transceiver to aid in acquisition by a first transceiver in the absence of a transmitted signal from the second transceiver. While a retro-reflector can aid target transceiver acquisition, it also can aggravate the crosstalk problem; i.e., the unwanted effects of a transmitted beam interfering with a received beam.
Crosstalk is a well-known problem inherent in systems having coaxial transmitter and receiver optics. Lang closes a mechanical shutter after acquisition to block the signal reflection from the second transceiver and thereby avoids crosstalk between a received message and the reflected transmitted signal. Smokler addresses the crosstalk problem by using separate frequencies, f.sub.1 and f.sub.2, to distinguish between a "standby" mode and a "call-up" mode. In the standby mode, each transceiver is permitted to transmit only f.sub.1 and to receive only f.sub.2. In the call-up mode, each transceiver is permitted to transmit only f.sub.2 and to receive only f.sub.1.
U.S. Pat. No. 3,504,182 issued to Pizzurro et al. reduces crosstalk by using a transceiver having noncoaxial transmitter and receiver optical axes. Similarly, in U.S. Pat. No. 4,867,560 issued to Kunitsugu, each satellite transceiver transmits a beam having a different wavelength. The received beam and a transmitted beam portion, which is split from the main beam by a dichroic mirror, impinge upon a four-quadrant sensor. When the satellites' transmitted beams are aligned, both beams impinge upon the center of the sensor. The output of the four-quadrant sensor represents beam divergence and is used as an error signal to adjust mirrors and realign the beams. This use of mirrors to steer the beams taught by Kunitsugu is very difficult to apply to a complete hemisphere scan, being more useful in situations where substantial alignment has already been achieved.
Various means have been suggested by practitioners in the art for maintaining optical communication where the beams are already substantially aligned. U.S. Pat. No. 3,942,894 issued to Maier uses servo-controlled optical elements that respond to sensors for aligning (and retransmitting) an incoming beam with a fixed reference annular mirror. U.S. Pat. No. 4,330,204 issued to Dye maintains alignment between two hand-held optical communicators by adjusting a spherical mirror in response to error signals from a quadrant sensor.
Other practitioners teach the use of optical communication systems that do not actively control alignment of the beams. U.S. Pat. No. 4,279,036 issued to Pfund discloses a satellite transceiver for use in communicating with a submarine. The transceiver includes a receiving array of detectors and a corresponding transmitting array of lasers. A particular laser element is selected according to which detector is energized by the beam transmitted from the submarine, which is presumed to know the satellite location. The beam then transmitted from the satellite illuminates the general area from which the submarine's beam was received. By illuminating a wide area, however, many of the advantages of a narrow beam are not realized.
U.S. Pat. No. 3,433,960 issued to Minott discloses a method that modulates and reflects a beam transmitted from a first ground station and incident upon the satellite's "retrodirective modulator" with modulation information extracted from a second beam incident on the satellite and transmitted from a second ground station. Minott relies on the ground stations to maintain beam alignment with the satellite.
U.S. Pat. No. 3,989,942 issued to Waddoups presents an Identification Friend or Foe (IFF) transponder that modulates and reflects an incoming beam. Unlike Minott, Waddoups includes means for active search and tracking of the ground station by the satellite using a Cassegrainian telescope and a quadrant sensor, although no specific methods for ground station acquisition are suggested.
Many problems remain unsolved in the art of reliable acquisition and maintenance of an optical communication link between two spatially isolated transceivers. Retro-reflectors have not heretofore been used successfully without undue crosstalk. Devices that maintain precise beam alignment have not been practically operated in harmony with telescopic acquisition means. These unresolved problems and deficiencies are clearly felt in the art and are solved by my invention in the manner described below.